The present invention relates to a digital filter bank for a voice multiplexing system.
The problem of designing a bank of bandpass filters occurs frequently in many signal processing systems, including spectrum analyzers and frequency division multiplexing (FDM) systems. Realization of such a filter bank is a particularly difficult task using analog techniques. The normal practice has been to design a standard bandpass filter with a suitable center frequency and to use a combination of frequency shifting and filtering to achieve the desired processing.
In a digital environment, it is not necessary to follow this practice. In theory, computing a discrete Fourier transform by use of a fast Fourier transform (FFT) or a Winograd Fourier Transform Algorithm (WFTA) together with a suitable weighting function could act as a bank of constant bandwidth filters.
A fast Fourier transform (FFT) is a computational technique of sequentially combining progressively larger weighted sums of data samples so as to produce discrete Fourier transform coefficients. The FFT technique can be interpreted in terms of combining the discrete Fourier transforms of individual data samples such that the occurrence times of these samples are taken into account sequentially and applied to discrete Fourier transforms of progressively larger mutually exclusive subgroups of data samples, which are combined to ultimately produce the discrete Fourier Transform of the complete series of data samples. A typical article describing the fast Fourier transform appears in IEEE Transactions on Computers, Nov. 1970, pp. 354-358.
Similarly, a Winograd Fourier Transform Algorithm (WFTA) is a technique for computing the discrete Fourier transform which significantly reduces the number of multiplication operations required. A typical article describing the Winograd Fourier Transform Algorithm appears in IEEE Transactions on Acoustics, Speech, and Signaling Processing, Vol. ASSP-25, No. 2, April 1977.
However, prior art systems utilizing digital filter techniques have generally not been capable of utilizing a hardware approach with fast Fourier or Winograd transform techniques as there are problems in attaining the necessary hardward complexity in order to utilize these transform techniques.
In view of the above background, it is desirable to provide a digital filter bank for voice multiplexing systems utilizing Fourier transform techniques such as described.